In the rat cerebral cortex and cerebellum large-scale cell proliferation, migration, and differentiation proceed during the first few weeks after birth. These cellular processes underlie the dynamic changes in brain morphology, electrophysiology and function that occur during postnatal develoment. The objectives of this project are to (a) define, for the first time, the normal pattern of gene activity in specific brain regions, and (b) characterize alterations in gene expression that occur during development. Estimates of the complexity of gene expression are obtained at the level of both transcription and translation by RNA/DNA hybridization under conditions of RNA excess. Regional and developmental comparisons in sequence complexity will be made using the nuclear RNA and polyadenylated mRNA populations. The possibility that a complex population of mRNA lacking poly(A) sequences may exist in brain is also being explored. The poly(A) mRNA copy frequency distribution will also be investigated using reassociation kinetic analysis of complementary DNA (cDNA). The results of this study should provide much needed insight into the role played by the genome in the development and maintenance of brain structure and function.